1. Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to wireless user equipment capable of uplink closed loop transmit diversity transmissions.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks.
As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications. For example, recent interest has been directed to uplink transmit diversity (ULTD) schemes, which employ more than one transmit antenna (usually two) at the user equipment (UE) to improve the uplink transmission performance: e.g., to reduce the UE transmit power, to increase the UE coverage range, to increase the UE data rate, or a combination of the above. ULTD can also help improve the overall system capacity. Based on the feedback requirements, ULTD schemes can be categorized into closed-loop (CL) and open-loop (OL) schemes. From the transmitter perspective, ULTD schemes can be classified as beamforming (BF) and antenna switching (AS) schemes.
In general, closed-loop transmit diversity (CLTD) schemes require the receiver to provide explicit feedback information about the spatial channel to assist the transmitter in choosing a transmission format over the multiple transmit antennas. One category of CLTD schemes is the CLTD beamforming scheme, where the base station (Node B) feeds back to the UE a precoding (or beamforming) weight vector to be applied to signals transmitted on the multiple transmit antennas so that the signals received at the Node B are constructively added. This in turn maximizes the receiver signal to noise ratio (SNR) and achieves the beamforming effect. However, due to the wide variety of implementations of CLTD schemes, there remains substantial developmental effort to improve and enhance these systems.